This invention relates to electrochemical systems for the production of chlorine and caustic having vacuum interrupter switches for cell shorting, and more particularly, to mercury cell electrochemical systems having vacuum interrupter switches.
According to one method of electrolyzing sodium chlorine brine to yield caustic soda and chlorine, the electrolysis is carried out in a mercury cell in which chlorine is liberated at the anode and the sodium is liberated at the cathode, the sodium forming an amalgam with the mercury. Mercury cells are characterized by the presence of a conducting surface inclined slightly from the horizontal in the longitudinal direction. A mercury amalgam film, typically from about 1/8 of an inch to about 1/4 of an inch or more in thickness flows across this plate in the direction of the inclination thereof. The flowing mercury amalgam film is the cathode.
Flowing on top of the amalgam is the electrolyte, that is, the aqueous sodium chloride solution. Typically, the electrolyte is of a thickness of from about 1/4 of an inch to about 2 inches or more at the inlet and as much as 10 or 12 inches at the outlet.
Anodes, such as carbon anodes or metallic anodes are usually spaced about 1/8 of an inch to about 3/16 of an inch above the surface of the mercury-amalgam film. In this way, electrical current can flow from the anodes through the electrolyte to the flowing mercury amalgam cathode. Structurally, a group of anodes are mechanically supported by and commonly movable on a frame or structural member which is electrically insulated from the cell body. Several anodes connected to a single conductor and arrayed laterally across the width of the cell are referred to as an anode row. A group of anode rows supported by a common frame or structural member are referred to as an anode bank.
In a typical electrolytic cell, there are a number of these anode banks arrayed along the longitudinal axis of the cell. For example, a typical cell may be 60 or 70 feet long and have anywhere from about 6 to about 30 anode rows arrayed along the length of the cell in about 4 to about 8 banks.
In a typical cell circuit, a plurality of individual cells are arranged in series. Typically, two or more rows of cells are in side-by-side relationship with the positive terminal of a power source being connected to the anodes of the end cell of the first row of cells and the negative terminal of the power source being connected to the cathodes of the end cell of the last row of cells. Within the series circuit, the cathodes of one cell are connected to the anodes of the next adjacent cell in the series. Typically, there may be from 30 to 80 cells in a cell circuit, although there may be more or less.
Cell by-pass switches are situated between the anode and the cathode conductors of each cell and are in the open position when the cell is in operation. When it is desired to shut off the current to a particular cell, the cell by-pass switch for that cell is placed in the closed position. Because the current path from the cell anode to the cell cathode (through the brine) has a much greater resistance than the current path through the switch, no current flows through the cell anode when the switch is in the closed position.
In the operation of mercury amalgam cathode cells, typically a very large current of from about 1000 amperes to about 120,000 amperes passes through the series of cells which operate at about a 5 volt potential difference between the anode and cathode of each cell. As a result, when the cell by-pass switch is operated, the large instantaneous change of current through the switch may give rise to an arc of considerable proportions across the switch contacts.
Typical cell by-pass switches generally comprise two elements: a switch driving mechanism, and one or more circuit interrupter assemblies. Throughout this specification, the following definitions will be used:
A circuit interrupter (also referred to herein as "interrupter") is the entity which contains the contact surfaces through which the electrical current passes when the surfaces are in contact (closed position).
A circuit interrupter assembly comprises one or more circuit interrupters mounted on a chassis. The one or more interrupters are connected in parallel and are mounted on the chassis in such a way that they open and close in concert when the drive mechanism is activated.
A cell by-pass switch comprises one or more "circuit interrupter assemblies" all in mechanical connection with a single drive mechanism so that the switch assemblies open and close in concert. Thus, the "cell by-pass switch" is the entire apparatus which performs the function of electrically by-passing an individual cell.
In prior art cell by-pass switches used in electrochemical systems, the circuit interrupters typically have contact surfaces which are open to the atmosphere and which are closed by the usual mechanical means. An example of one such circuit interrupter is the Burndy Type HSG High Current Switch sold by Burndy Co., Norwalk, Conn. 06856. The use of such circuit interrupters on electrochemical cells has several drawbacks.
First, the contact surfaces of the interrupters exposed to the environment and atmosphere of the cell rooms tend to become coated with foreign materials from brine and caustic, which may leak from the cells onto the interrupters, or by chlorine gas which may be present in the atmosphere. As a result, conducting surface contact will be less than complete, and the presence of high resistance foreign materials between the contact surfaces may cause the interrupter to become hot when it is in the closed position. The presence of resistive matter on the contact surfaces of the interrupter can also prevent it from effectively shutting off current going to the anode, thus necessitating the premature replacement of the interrupter. A second problem with interrupters having contacts open to the atmosphere is that arcing takes place whenever the switch is opened or closed. This phenomenon is intensified when the arc occurs in the ambient air and increases pitting of the contact surfaces, thus, shortening the operational lifetime of the interrupter. Finally, there is the possibility that an open arc may ignite a flammable gas, such as H.sub.2, which may be present in the plant atmosphere.
In an electrochemical system, the problems listed above are aggravated because the cells are connected in series. Thus, the power to all of the cells in the plant must be shut off in order to replace an interrupter that has become non-operational.